The present invention relates to a transparent coordinate detection device. Specifically, the present invention relates to a transparent tablet of a transparent coordinate detection device wherein the lead wires are integrally formed with the transparent resistor film.
Transparent membrane touch panels for use with the screen of a cathode ray tube are well known in the prior art. The heart of the system is the coordinate detection substrate, which includes a lower transparent plate oriented in one direction, and an orthogonally oriented upper transparent plate. The application of pressure to the upper plate forces it into electric contact with the lower, which enables external systems to determine the coordinates of the applied pressure.
A conventional transparent tablet of a coordinate detection substrate according to the prior art is shown in FIGS. 3-6. Referring to FIG. 3, an X-axis resistance plate 2 has a roughly rectangular shape. A first pair of electrodes 1A and 1B are formed along the ends of X-resistance plate 2 in a X-direction 17. Conductors 6 and 7 connect electrodes 1A and 1B, respectively, to their respective contacts X1, X2, and Y3.
A Y-axis resistance plate 5 is positioned below X-resistance plate 2. A second pair of electrodes 4A and 4B are placed along a Y-axis direction 16 of Y-axis resistance plate 5. A small space is maintained between the plates by non-conductive dot spacers 3. Conductors 8 and 9 connect electrodes 4A and 4B to their respective contacts Y1, Y2 and X3.
A switch 11 selectively applies current from a power supply 10 to either first pair of electrodes 1A and 1B or second pair 4A and 4B. Switch 11 also has a pole connected to an input of an analog to digital converter 12 which converts the voltages between selected pairs of electrodes into digital signals. An operation circuit 13 converts these digital signals into the X and Y coordinate values.
Referring now to FIG. 4, an X-axis resistance plate 2 of a transparent tablet has a substrate 2A upon which a transparent resistor 2B has been formed as a uniform thin film. First pair of electrodes 1A and 1B, usually silver or other conductive metals, are placed apart on transparent resistor 2B at predetermined intervals. A pair of lead wires 6A and 7A, which are made from the same material as the first pair of electrodes, 1A and 1B, connect each electrode 1A and 1B to conductors 6 and 7, respectively.
The conventional transparent coordinate detector as described above operates as follows. Referring now to FIG. 3, switch 11 is set at positions X1, X2 and X3, and current is supplied to first pair of electrodes 1A and 1B from power source 10. Referring now to FIG. 4, equipotential lines represent a potential gradient created on transparent resistor 2B between electrode 1A, which is at a high potential, and electrode 1B, which is at a low potential.
Referring now to FIG. 5, a pen 14 is applied against a desired portion of the surface of X-axis resistance plate 2. The pressure causes the X-axis resistance plate to bend against dot spacers 3, resulting in electrical contact with Y-axis plate 5. Once the short circuit is achieved, the resistances of circuit elements Rx1-Rx6 form a voltage divider in the equipotential lines previously formed between electrodes 1A and 1B.
Referring once again to FIG. 3, the resistances between the location of pen 14 and the first pair of electrodes 1A and 1B are Rx1 and Rx2, respectively. The contact resistances between conductors 6 and 7 and X-axis resistance plate are Rx3 and Rx4, respectively. The resistances of the individual conductors is Rx5 and Rx6. The voltage divider thus created by the pen is given by the equation: ##EQU1##
V.sub.Ref is input into A/D converter 12 as the X-axis coordinate information via the Ry2 side of Y-axis resistance plate 5. A/D converter 12 also receives power supply 10 as an input through a variable resistor 15. Once the above two signals are digitized, operation circuit 13 calculates the X-axis coordinate location depressed by pen 14 in accordance with the above equation.
With pen 14 in the same position, switch 11 switches from the X contacts to the Y contacts at Y1, Y2 and Y3. In a similar manner to X-axis resistance plate 2, a voltage divider is created by pen 14 as follows: ##EQU2##
V.sub.Ref is input to A/D converter 12 through Rx2. V.sub.Ref and power supply 10 are digitized by A/D converter 12, which in turn is processed by operation circuit 13 to produce the corresponding Y coordinate value.
A/D converter 12 and operation circuit 13 present a high input impedance, which neutralizes their influence on the coordinate measurements by Rx2 and Ry2 and on the contact resistance between X-axis resistance plate 2 and Y-axis resistance plate 5.
A drawback of the prior art is that electrodes 1A and 1B and lead wires 6A and 7A are made of silver, which results in high production and material costs.
In order to overcome the above drawback, it has been suggested to replace the conductive silver in the device with integrally forming lead wires 6A and 7A of the same material as transparent resistor 2B. Such a configuration is shown in FIG. 6, in which lead wires 6B and 7B are integrally formed with transparent resistor 2B.
A drawback of the above suggestion is that the transparent resistor material has a higher resistance than silver. The absence of a low resistance equipotential silver electrode across ends of transparent resistor 2B results in a non-linear potential gradient. In the case of the uniformly formed plate of FIG. 6, the resistance along the edges is dependent upon the distance from the points where the integrally formed lead wires meet the resistor plate. As a result, a warped potential gradient, shown by field lines V, is produced which prevents accurate coordinate calculations.